UPS is hiring 90,000 to 95,000 seasonal workers to keep up with the deluge of holiday package deliveries. / Mark Lennihan, AP

by Donna Leinwand Leger , USATODAY

by Donna Leinwand Leger , USATODAY

Thousands of holiday gifts won't arrive by Christmas after an unpredictably large number of packages overwhelmed UPS, the world's largest package delivery service.

"The volume of air packages in our system exceeded the capacity of our network, as demand was much greater than the forecast," UPS spokeswoman Natalie Godwin said Tuesday. "As a a result a small percentage of shipments are delayed and will not be delivered today."

Godwin would not say how many packages had been delayed, but the delays are nationwide.

UPS drivers will deliver packages on Christmas Eve, but will not deliver on Christmas Day. Workers at the company's Louisville hub will work on Wednesday night to sort packages so they can be delivered on Thursday. she said.

"We are using every available resource to get packages delivered," she said.

Customers hit by delays carped online and posted their tracking details on Twitter.

@LaneDouglas of Bethesda, Md. wrote "#Amazon Prime doesn't mean jack when #UPS tells you they had a carrier delay."

A woman in Saginaw, Tex., posted a tracking receipt that showed an order placed with The Disney Store on Dec. 17 won't arrive until after Christmas even though the items left Tennessee and arrived in nearby Fort Worth, Tex. at 8:05 p.m. Dec. 20.

UPS last week reported delays in the Dallas-Fort Worth area after severe weather hit the region. Godwin says the majority of customers who received notices last week of delays should receive their packages today. UPS does not guarantee Christmas delivery for ground shipping after Dec. 11.